Stateless call admission and call preemption with a single metering and marking scheme

ABSTRACT

A method and system for call admission and preemption in a network having an ingress node and an egress node defining an ingress/egress node pair, and one or more core nodes located in a path between the ingress node and the egress node and configured to mark packets exceeding an admission threshold. The method includes calculating a sustainable preemption rate based on a rate of traffic received at the egress node from the ingress node that passed through the one or more core nodes without being marked, and a ratio between a preemption threshold and the admission threshold. If a traffic load to the egress node is greater than the sustainable preemption rate, at least some traffic is dropped so that the traffic load does not exceed the sustainable preemption rate for the ingress/egress node pair.

STATEMENT OF RELATED APPLICATION

The present application claims priority from U.S. ProvisionalApplication No. 60/813,674, entitled STATELESS CALL ADMISSION AND CALLPREEMPTION WITH A SINGLE METERING AND MARKING SCHEME, and filed on Jun.14, 2006 (Attorney Docket No. CISCP915+). The contents of thisprovisional application are incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

The present disclosure relates generally to stateless call admission andcall preemption with a single metering and marking scheme.

The IETF (Internet Engineering Task Force) is investigating methods forcontrolling the admitted load of voice (and multimedia) traffic over apacket network without support of per flow, or even per aggregate,states in the core. (See, B. Briscoe et al., “A Framework for AdmissionControl over DiffServ using Pre-Congestion Notification”, IETFdraft-briscoe-tsvwg-cl-architecture-03.txt, Jun. 26, 2006 and B. Briscoeet al., “Pre-Congestion Notification Marking”, IETFdraft-briscoe-tsvwg-cl-phb-02.txt, Jun. 26, 2006, which are bothincorporated by reference herein in their entirety). This can bereferred to as a CAC (Call Admission Control) approach over a statelesscore.

The method relies on core nodes providing explicit notificationinformation to edge nodes when the traffic load on a core link reachescertain levels. The edge nodes use some of this explicit notificationinformation to decide whether new calls should be accepted. This isreferred to as a flow “Admission” process. The edge nodes may also usesome explicit notification information to drop calls that are already inplace if needed. This is referred to as a flow “Preemption” process.

In order to provide the explicit notification information, the core nodemeters voice traffic load on each link, and when it reaches predefinedthresholds the core node sets particular bits in the header of packets(PCN-bits (pre-congestion notification-bits)).

An important requirement for this method is that the Admission andPreemption processes are able to react at different load thresholds. Forexample, a network operator policy may be to stop admitting new voicecalls when the load through a given link reaches 50%, but only startdropping calls when the load exceeds 70%.

Because of this requirement, the solutions proposed in IETF typicallyinvolve separate metering/marking schemes for Admission and forPreemption. In core routers, the traffic is metered against theAdmission Threshold, and if in excess, a codepoint is set in thePCN-bits indicating “Admission Threshold exceeded”. The traffic is alsometered against the Preemption Threshold, and if in excess, anothercodepoint is set in the PCN-bits indicating “Preemption Thresholdexceeded”.

One approach proposed in IETF does not require preemption marking,however, it does require two separate measurement schemes; onemeasurement for Admission and another measurement forPreemption/Dropping. Furthermore, this approach mandates that theconfigured preemption rate is set to a drop rate. This is a significantrestriction in that it results in preemption only taking effect oncepackets actually get dropped.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a portion of an example network in which embodimentsdescribed herein may be implemented.

FIG. 2 illustrates measurement of Sustainable Admission Rate by egressedge nodes of the network of FIG. 1.

FIG. 3 illustrates an example of reporting of the Sustainable AdmissionRate by the egress edge nodes to ingress edge nodes in the network ofFIG. 1.

FIG. 4 illustrates an example of calculation of a Sustainable PreemptionRate by the ingress edge nodes.

FIG. 5 illustrates an example of reduction of the current load to theSustainable Preemption Rate calculated in FIG. 4.

FIG. 6 is a flowchart illustrating one example of a process forstateless call admission and preemption with a single metering andmarking scheme.

FIG. 7 depicts an example of a network device useful in implementingembodiments described herein.

DESCRIPTION OF EXAMPLE EMBODIMENTS Overview

A method and system for call admission and preemption in a networkhaving an ingress node and an egress node defining an ingress/egressnode pair, and one or more core nodes located in a path between theingress node and the egress node and configured to mark packetsexceeding an admission threshold. The method includes calculating asustainable preemption rate based on a rate of traffic received at theegress node from the ingress node that passed through the one or morecore nodes without being marked, and a ratio between a preemptionthreshold and the admission threshold. If a traffic load to the egressnode is greater than the sustainable preemption rate, at least sometraffic is dropped so that the traffic load does not exceed thesustainable preemption rate for the ingress/egress node pair.

Example Embodiments

The following description is presented to enable one of ordinary skillin the art to make and use the invention. Descriptions of specificembodiments and applications are provided only as examples and variousmodifications will be readily apparent to those skilled in the art. Thegeneral principles described herein may be applied to other embodimentsand applications without departing from the scope of the invention.Thus, the present invention is not to be limited to the embodimentsshown, but is to be accorded the widest scope consistent with theprinciples and features described herein. For purpose of clarity,details relating to technical material that is known in the technicalfields related to the invention have not been described in detail.

A system and method described herein use separate Admission andPreemption Thresholds while using a single metering/marking scheme toenforce both. As described in detail below, the system and method reduceimplementation requirements on core routers and reduce the number ofcodepoints needed in a very scarce resource (PCN-bits in packet header).The system and method described herein provide these benefits whileimposing reasonably acceptable deployment constraints.

The embodiments described herein allow a system with a singlemetering/marking scheme to behave similarly to a system that usesseparate metering/marking schemes for Admission and for Preemption, andis configured on all links with Preemption Thresholds and AdmissionThresholds in the same ratio. As described in detail below, the systemfirst uses a rate-based metering scheme for flow Admission and thenimposes a system-wide restriction on the ratio between the configuredAdmission Threshold and Preemption Threshold.

Referring now to the drawings, and first to FIG. 1, one example of anetwork that may implement embodiments described herein is shown. Forsimplification, only a small number of nodes are shown. The embodimentsoperate in the context of a data communication network includingmultiple network elements. Some of the nodes in a network that employsthe embodiments may be network devices such as routers or gateways. Thenetwork device may include, for example, a master central processingunit (CPU), interfaces, and a bus. The CPU preferably includes memoryand a processor. The network device may be implemented on a generalpurpose network host machine such as a computer system or network devicedescribed below with respect to FIG. 7.

Data traffic flows through various nodes including edge nodes 10, 12,14, 16 and core nodes 18, 20. Adjacent nodes are coupled via one or morecommunication paths (links). The edge nodes may be ingress or egressedge nodes, depending on the direction of the traffic flow. As describedbelow, the core nodes 18, 20 meter traffic and mark packets as required.The edge nodes 10, 12, 14, 16 use the metering and marking to performflow Admission.

The ingress and egress edge nodes define ingress/egress node pairs. Forexample, in the network shown in FIG. 1, one ingress/egress node paircomprises nodes 10 and 14 and another ingress/egress node pair comprisesnodes 12 and 16. The measurement and feedback described below areperformed for each ingress/egress pair. For example, if two ingressnodes communicate to the same egress node, the egress node takes twomeasurements, one for each ingress node, and reports the correspondingmeasurement separately to each ingress node. Similarly, if one ingressnode is in communication with two egress nodes, each egress node reportsits own ingress/egress node pair measurement to the ingress node.

It is to be understood that the network shown in FIG. 1 is just oneexample and that the method and system described herein may be usedwithin various configurations, sizes, and types of networks. The methodand system may be implemented, for example, in network edge routers,VoIP Gateways, or Session Border Controllers (SBCs).

FIG. 1 illustrates the state of the network prior to preemption. Trafficpasses along path 22 from ingress edge node 10 through core (interiornodes) 18, 20 to egress edge node 14. Traffic is also routed fromingress edge node 12 to egress edge node 16 through core nodes 18 and 20via path 24. In the example of FIG. 1, the traffic load (bandwidth,rate) before preemption is B₁=6 at edge node 10 and B₂=10 at edge node12. The total load passing from node 18 to node 20 is B_(Total)=6+10=16.The total load may be greater than the admission threshold of 10 for anumber of reasons, such as rerouting due to link failure, for example.

The system preferably utilizes a metering scheme (e.g.,Token-bucket+excess marking, or other suitable metering scheme) that isable to convey to the egress edge node the rate of traffic (for a giveningress/egress pair) that traverses the core while being under themetered rate on every link. In one embodiment, the core routers 18, 20meter traffic (e.g., voice traffic) against a single Token Bucket whoserate is configured to the Admission Threshold. The core routers 18, 20mark all packets which exceed the Token Bucket with an “AdmissionThreshold Exceeded” codepoint. The edge nodes may use this Admissionmetering and marking to perform flow Admission as defined, for example,in IETF draft-briscoe-tsvwg-cl-architecture-03.txt (referenced above).

In the example depicted in FIG. 1, the Admission Threshold (Admit) isset to 10 and the implicit Preemption Threshold (Implicit Preempt) isset to 14. These Threshold values may be set, for example, by a networkoperator in accordance with a predefined policy. A system-widePreemption/Admission ratio (R) is defined as PreemptionThreshold/Admission Threshold. The ratio (R) between the PreemptionThreshold and Admission Threshold is the same on all of the links in thecore. In the example of FIG. 1, the Preemption/Admission ratio(R)=14/10.

In the example of FIG. 1, since the total load passing from node 18 tonode 20 is B_(Total)=6+10=16 while the Admission Threshold on that linkis set to 10, the proportion of traffic that is forwarded by core router18 onto that link unmarked is (Admission Threshold/B_(Total))=10/16. Theproportion of traffic forwarded onto that link with marking of the“Admission Threshold Exceeded” codepoint by core router 18 is((B_(Total)−Admission Threshold)/B_(Total))=6/16.

FIG. 2 illustrates how the egress edges measure a Sustainable AdmissionRate (SA) (i.e., rate of traffic which went through the core withoutbeing marked with Admission Threshold exceeded codepoint). At egressedge nodes 14 and 16, SA is calculated by measuring the rate of trafficreceived unmarked (i.e., without the “Admission Threshold Exceeded”codepoint set). Since, as explained above, the proportion of trafficforwarded unmarked by core router 18 is 10/16, the rate of trafficreceived unmarked at egress node 14 from ingress node 10, and at egressnode 16 from ingress node 12, is, respectively:

SA _(10/14) =B ₁*10/16=6*(10/16)=3.75

SA _(12/16) =B ₂*10/16=10*(10/16)=6.25

-   -   where:        -   SA_(10/14) is the Sustainable Admission Rate for traffic            from ingress node 10 to egress node 14 (e.g., for            ingress/egress node pair 10/14); and        -   SA_(12/16) is the Sustainable Admission Rate for traffic            from ingress node 12 to egress node 16 (e.g., for            ingress/egress node pair 12/16).

The system is preferably configured to provide conservative notificationof the ingress node by the egress node that preemption may be needed(and provision of sufficient information to the ingress node to decide),to compensate for the fact that the egress node does not receive anyexplicit notification about a Preemption Threshold being exceeded in thecore (unlike when a separate Preemption metering and marking scheme isused).

When the egress edge node 14, 16 detects that the Admission Threshold isexceeded (for a given ingress/egress pair), the egress node reportsfrequently to the ingress nodes 10 and 12 the measured rate of trafficfrom the corresponding ingress to the corresponding egress that wentthrough the core without being marked with Admission Threshold exceededcodepoint.

FIG. 3 illustrates the egress edge nodes 14, 16 signaling to the ingressedge nodes 10, 12 to report the measured rate of traffic that wentthrough the core without being marked (Sustainable Admission Rate).Egress node 14 reports SA_(10/14) to ingress node 10 and egress node 16reports SA_(12/16) to ingress node 12, as depicted by dashed lines 26,28, respectively.

As those skilled in the arts will appreciate, an egress node, e.g., 14,may receive traffic from multiple ingress nodes, e.g., 10, 12. In thiscase, the egress node measures a separate Sustainable Admission Ratefrom each ingress node, and reports it separately to each ingress node.In effect the system uses a separate Sustainable Admission Rate for eachingress/egress node pair.

Since the ratio (R) between the Preemption Threshold and AdmissionThreshold is the same on all links in the core, the rate of traffic fora given ingress/egress pair that traverses the core under the PreemptionThreshold can be approximated by multiplying by R the SustainableAdmission rate for that ingress/egress pair. FIG. 4 illustrates how theingress edge nodes 10, 12 compute a Sustainable Preemption Rate (SP) bymultiplying the Sustainable Admission Rate (SA) by the system-widePreemption/Admission ratio (R). SP is calculated as follows for theexample shown in FIG. 4:

SP _(10/14) =SA _(10/14) *R=3.75*(14/10)=5.25

SP _(12/16) =SA _(12/16) *R=6.25*(14/10)=8.75

FIG. 5 illustrates how the ingress edge nodes 10, 12 drop some flowsaccording to a predefined policy to reduce the current load to theSustainable Preemption Rate (SP) (modulo some safety/hysteresis factor).In this example, B₁ is reduced from 6 to 5.25 and B₂ is reduced from 10to 8.75. The total rate B_(Total) on the link from node 18 to node 20 isnow equal to 14, which is the Implicit Preemption Rate on that link.

The following describes one example of admission control with the singlemarking and metering scheme in the core. The egress edge nodes measurethe admission marked traffic or the total traffic in order to measureCongestion Level Estimate (CLE). CLE is the ratio of admission-markedtraffic to the sustainable rate. The admission control decision is basedon the CLE. The ingress stops admitting when the CLE level is above aconfigurable threshold. For example, the ingress edge node may stopadmitting traffic to a specified egress edge node when it receives a CLEover 1% of the sustainable rate to that egress edge node.

A flowchart illustrating one embodiment of the process described abovefor stateless call admission and call preemption with a single meteringand marking scheme is shown in FIG. 6. At step 50, the core routersmeter traffic and mark all packets that exceed the Admission Threshold.The egress edge nodes then measures the Sustainable Admission Rate (SA)for each ingress edge based on the measured rate of packets that wentthrough the core without being marked (step 52). The egress edge nodesreport the relevant Sustainable Admission Rate to each ingress edgenode. The Sustainable Preemption Rate (SP) is then calculated at theingress edge nodes for each egress node (step 56). The ingress edgenodes drop some flows traveling through a given egress node if thecurrent load to that egress is greater than the Sustainable PreemptionRate (SP) calculated at step 56 for that egress (steps 58 and 59).

It is to be understood that the method illustrated in FIG. 6 anddescribed above is only one example and that one or more of the stepsmay be performed at different nodes, without departing from the scope ofthe invention. For example, the egress node may compute the SustainablePreemption Rate and signal the Sustainable Preemption Rate to theingress node, rather than signaling the measured Sustainable AdmissionRate. Also, the egress node may preempt flows. One or more of the stepsmay also be performed at a node in the network other than the ingress oregress nodes.

FIG. 7 depicts a network device 60 that may be used to implementembodiments described herein. In one embodiment, network device 60 is aprogrammable machine that may be implemented in hardware, software, orany combination thereof. A processor 62 executes codes stored in aprogram memory 64. Program memory 64 is one example of acomputer-readable medium. Program memory 64 can be a volatile memory.Another form of computer-readable medium storing the same codes would besome type of non-volatile storage such as floppy disks, CD-ROMs,DVD-ROMs, hard disks, flash memory, etc. A carrier wave that carries thecode across the network is an example of a transmission medium.

Network device 60 interfaces with physical media via a plurality oflinecards 66. Linecards 66 may incorporate Ethernet interfaces, DSLinterfaces, Gigabit Ethernet interfaces, 10-Gigabit Ethernet interfaces,SONET interfaces, etc. As packets are received, processed, and forwardedby network device 60, they may be stored in a packet memory 68. Toimplement functionality according to the system, linecards 66 mayincorporate processing and memory resources similar to those discussedabove in connection with the network device as a whole.

As can be observed from the foregoing, the system and method describedherein provide numerous advantages. For example, the use of a singlemetering/marking scheme for both Admission and Preemption provide anumber of benefits, including reduced implementation requirements oncore routers. Only one metering (e.g., token bucket) and marking schemeneed to be activated on core routers and each packet only needs to bemetered against this single metering.

Also, the single metering/marking scheme allows for a reduced number ofcodepoints that need to be conveyed in the packet header. The PCN-bitsproposed to be used in the packet header to convey the congestionnotification information are the bits of the ECN field (explicitcongestion notification) (defined in S. Bradner et al., “IANA AllocationGuidelines For Values In the Internet Protocol and Related Headers”, RFC2780, March 2000 and K. Ramakrishnan et al., “The Addition of ExplicitCongestion Notification (ECN) to IP”, RFC 3168, September 2001) in an IPcore and the bits of the EXP field (experimental use) (defined in E.Rosen et al., “MPLS Label Stack Encoding”, RFC 3032, January 2001) in anMPLS core. These are small fields which are already targeted formultiple use and applications. For example, additional codepoints needto be conveyed in the same field (e.g., for anti-cheating and perhapsfor compatibility with non-CAC capable systems), currently bringing thetotal to potentially 5 codepoints in systems using separate markingschemes for Admission and Preemption. The field available in case of IPis only 2-bits (i.e., the ECN field). In MPLS there is only a total of 8EXP codepoints which must be shared with Diffserv. Thus, eliminating acodepoint is advantageous. Even if other bits than the ECN field or EXPbits were to be used in the packet header as the PCN-bits, there arevery little bits or codepoints remaining available anywhere in thepacket header, so eliminating a codepoint would be equally advantageous.

Furthermore, the system and method require only one measurement foradmission, rather than one measurement for admission and anothermeasurement for preemption/dropping. Also, there is no need for theconfigured preemption rate to be set to the drop rate.

Although the method and system have been described in accordance withthe embodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations made to the embodiments withoutdeparting from the scope of the present invention. Accordingly, it isintended that all matter contained in the above description and shown inthe accompanying drawings shall be interpreted as illustrative and notin a limiting sense.

1. A method for call admission and preemption in a network comprising aningress node and an egress node defining an ingress/egress node pair,and one or more core nodes located in a path between the ingress nodeand the egress node and configured to mark packets exceeding anadmission threshold, the method comprising: calculating a sustainablepreemption rate based on a rate of traffic received at the egress nodefrom the ingress node that passed through the one or more core nodeswithout being marked, and a ratio between a preemption threshold andsaid admission threshold; and if a traffic load to the egress node isgreater than said sustainable preemption rate, dropping at least sometraffic so that said traffic load does not exceed said sustainablepreemption rate for the ingress/egress node pair.
 2. The method of claim1 wherein said ratio between said preemption threshold and saidadmission threshold applies to other ingress and egress node pairshaving a communication path passing through the one or more core nodes.3. The method of claim 1 wherein the one or more core nodes areconfigured to meter traffic against a token bucket having a rate set tosaid admission threshold.
 4. The method of claim 1 wherein said rate oftraffic is not based on dropped or preempted packets.
 5. The method ofclaim 1 further comprising: receiving a congestion level estimate basedon admission marked traffic at the egress node; and stopping admittingof traffic to the egress node if said congestion level estimate is overa specified limit.
 6. The method of claim 1 wherein calculating asustainable preemption rate is performed at the ingress node.
 7. Themethod of claim 1 wherein dropping at least some traffic is performed atthe ingress node.
 8. The method of claim 1 further comprising signalingsaid rate of traffic from the egress node to the ingress node.
 9. Themethod of claim 1 wherein calculating said sustainable preemption rateis performed at the egress node and further comprising signaling saidsustainable preemption rate from the egress node to the ingress node.10. The method of claim 1 wherein dropping at least some traffic isperformed at the egress node.
 11. Apparatus for call admission andpreemption, the apparatus configured for operation in a networkcomprising an ingress node and an egress node defining an ingress/egressnode pair, and one or more core nodes located in a path between theingress node and the egress node and configured to mark packetsexceeding an admission threshold, the apparatus comprising: a processor;and a memory that stores instructions for execution by said processor,said instructions comprising: code that calculates a sustainablepreemption rate based on a rate of traffic received at the egress nodefrom the ingress node that passed through the one or more core nodeswithout being marked and a ratio between a preemption threshold and saidadmission threshold; and code that, if a traffic load to the egress nodeis greater than said sustainable preemption rate, causes dropping of atleast some traffic so that said traffic load does not exceed saidsustainable preemption rate for the ingress/egress node pair.
 12. Theapparatus of claim 11 wherein the apparatus is configured for receivinga congestion level estimate based on admission marked traffic at theegress node, and to stop admitting traffic to the egress node if saidcongestion level estimate is over a specified limit.
 13. The apparatusof claim 11 wherein said ratio between said preemption threshold andsaid admission threshold is configured to apply to other ingress andegress node pairs having a communication path passing through the one ormore core nodes.
 14. Apparatus for call admission and preemption in anetwork comprising an ingress node and an egress node defining aningress/egress node pair, and one or more core nodes located in a pathbetween an ingress node and an egress node and configured to markpackets exceeding an admission threshold, the apparatus comprising:means for calculating a sustainable preemption rate based on a rate oftraffic received at the egress node from the ingress node that passedthrough the one or more core nodes without being marked, and a ratiobetween a preemption threshold and said admission threshold; and if atraffic load to the egress node is greater than said sustainablepreemption rate, means for dropping at least some traffic so that saidtraffic load does not exceed said sustainable preemption rate for theingress/egress node pair.
 15. The apparatus of claim 14 wherein the oneor more core nodes are configured to meter traffic against a tokenbucket having a rate set to said admission threshold.
 16. The apparatusof claim 14 wherein said rate of traffic is not based on dropped orpreempted packets.
 17. The apparatus of claim 14 further comprising:means for receiving a congestion level estimate based on admissionmarked traffic at the egress node; and means for stopping admitting oftraffic to the egress node if said congestion level estimate is over aspecified limit.
 18. The apparatus of claim 14 wherein the apparatus isconfigured to operate as the ingress node or the egress node and saidratio is a predefined ratio that applies to other ingress/egress nodepairs in the network.